The present invention relates in general to an apparatus for powering X-ray tubes. In particular, the present invention uses solid-state switching and closed loop feedback control to automatically control the X-ray exposure. The invention may be correctly used by operators having a minimal skill level.
Typical high-voltage power supplies utilize vacuum tubes, variacs, auto transformers, tetrodes, and/or triodes for regulating the high-voltage output supplied to an X-ray tube. These devices have not been totally satisfactory due to their relatively high cost, slow response speed, lack of reliability, and/or their inability to provide a pulsed waveform having minimum ripple, fast rise time, and fast fall time. Prior attempts have been made to utilize solid-state switching devices, such as a series of field effect transistors (FETs), to regulate the high-voltage output. However, these have met with limited success because of the low-power rating of commercially available transistors and their inability to survive voltage spikes caused by an X-ray tube arc-over.
In a conventional X-ray tube, X-rays are produced by generating electrons by thermionic emission from a tungsten filament (cathode). The electrons are then accelerated to an anode (which may be rotating for wear-averaging purposes) to generate the X-rays. The emission intensity of the tube is controlled by the filament current and by the difference in potential between the anode and cathode.
Current X-ray tubes operate at potentials of up to 200 kV. A high-voltage power supply utilizing a step-up transformer is required to raise the available AC line voltage to this level. X-ray tube power supplies having a DC supply circuit, 90, as shown in FIG. 1 have been employed in prior known devices. The typical AC line voltage available in hospitals and clinics varies from single-phase 220 volts AC to three-phase 600 volts AC. An X-ray power supply able to operate from widely different line voltages, phases and impedances would be desirable in that line matching of the transformer to the specific AC line voltage characteristics would not be required.
Precise control of the voltage and phase of the power supplied to an X-ray tube is important to ensure proper imaging for diagnostic purposes and to avoid unnecessary exposure of the patient to X-ray radiation which does not produce a useable image. For example, during a conventional radiographic gastrointestinal analysis, the patient ingests a radioopaque liquid containing barium. When the patient ingests the liquid, the doctor turns on the X-ray generating tube at a low level and positions the patient between the X-ray tube and a fluoroscopic screen. The doctor analyzes the patient's gastrointestinal tract while the barium flows through it. When the doctor sees a part of the procedure he wants to record, he typically replaces the fluoroscopic screen with a photographic plate and increases the X-ray to a level intense enough to expose the plate.
High operating voltages make control of the X-ray tube emission level a difficult problem, typically requiring expensive components. Furthermore, at high tube currents the voltage can fall very quickly, making precise measurement of the voltage difficult. Still further, at high voltages stray capacitive coupling occurs which prevents accurate measurement of the tube voltage. Regulator circuits using FET's in series in X-ray tube power supplies have been employed in prior art devices. The combination of FET's in series regulator circuits with the protection, drive and feedback circuits of the embodiment of FIG. 1 is quite novel, however, and the specific configuration of the regulator circuit 91, FIG. 1, is also quite novel.
Precise phase control is also important when an X-ray image is to be recorded by a television camera. TV cameras have well-established sweep rates to which the X-ray exposure must be synchronized. If the exposure is not synchronized, the resulting picture from the TV camera has an interference pattern or jitters, which will make the picture very difficult or impossible to view. An exposure synchronized with the 60 Hz sweep rate of the TV camera will produce a coherent picture. It is also permissible to use X-ray exposures of less frequent multiples of the 60 Hz rate, for example, 30, 15 or 7.5 Hz.
Single-phase and three-phase power supplies each have certain advantages, depending upon the exposure rate desired. For example, three-phase power supplies are commonly employed to provide continuous X-ray emissions, because the voltage ripple in the rectified signal is smaller. Filtering capacitors to eliminate ripple are generally impractical at the high voltages employed and interfere with switching on-off times. In contrast, single-phase supplies are generally used to provide a short pulsed emission.
It would thus be desirable to provide a high-voltage power supply for both continuous and pulsed X-ray emissions which provides a precisely controlled output voltage and which accommodates variations in the input AC line voltage.
When an X-ray tube is being used for applications such as cardiac angiography, the X-ray tube is operated in relatively short bursts at a relatively high frequency in order to obtain clear images and to be able to monitor heart activity and detect any abnormalities. Typically, the tube is operated at approximately 8 ms bursts.
The relatively small X-ray tube current produced with pulsed fluroscopy does not sufficiently discharge the capacitance of the high-voltage cables connecting the power supply and X-ray tube between exposure frames. The "tail" on the power supply output waveform produces unwanted soft radiation which adds to the patient dose and does not improve the image. It would thus be desirable to provide a high-voltage power supply for an X-ray tube which produced substantially rectangular waveforms without a trailing tail of unwanted soft radiation.
In a conventional technique for measuring the X-ray tube current, the anode and cathode power supply lines are separated, with one supply return tied to ground and the other supply return tied to ground through a small value resistor in order to measure the "midpoint" current. However, the accuracy of this measurement technique is adversely affected by high-tension transformer leakage current and various stray capacitive currents. It would be desirable to provide an apparatus for precisely measuring X-ray tube current which is not affected by these leakage and stray capacitive currents.
It is generally difficult to measure the true X-ray tube current with a measuring circuit placed in either the anode or the cathode circuit because these circuits are both at potentials of up to 100 kV away from ground and there is no low-voltage power supply available. It would be desirable to provide a measuring circuit which is powered by the X-ray tube current it is measuring and which is capable of sending accurate information across the 100 kV boundary.
Still further, although X-ray tube currents for pulsed fluoroscopy may be relatively small, i.e., 0.5 milliamps (ma), X-ray tube currents may be as high as 1200 ma during full exposures. The X-ray tube current thus ranges from 0.5 ma to 1200 ma. As a result, the measuring circuit must be linear over a 2400 to 1 dynamic range. It would be desirable to provide a current measuring apparatus which is linear over this dynamic range and which operates with only 0.5 ma available as a supply current.
As noted above, the use and control of high-voltage power is inherent in the operation of conventional X-ray tubes. As such, various circuit components which are the same or similar to a variety of individual components shown in the Figures such as voltage divider 5, filament drive 73, current measurer 320 and discharge modules 210A, B, FIG. 1, have been employed in prior X-ray power supply devices. The combination of such components and the configuration of such components and the configurations of elements such as 210 as in the present invention is quite novel, however.
Thus, it is an object of this invention to provide a high-voltage regulated power supply for an X-ray tube that provides a precisely controlled voltage waveform in order to avoid overexposing the patient to excessive radiation or underexposing the X-ray image.
Another object is to provide a regulated power supply utilizing solid-state switching devices having a fast response time to enable real time control of the patient X-ray dosage.
Yet another object is to provide a regulated power supply having protective circuitry to guard against damage from excessive current or voltage transients.
Still another object is to provide an X-ray apparatus that automatically adjusts X-ray exposure without the aid of an X-ray technician to assure properly exposed X-ray images for patients having widely varying body sizes.
Still another object is to provide an X-ray apparatus that can automatically adjust X-ray exposure during the actual exposure time.
Still another object is to provide an X-ray apparatus in which X-ray tube current and voltage may be adjusted under load.